Neptune facts and information

One of the largest, and slowest-moving, planets in the solar system, Neptune is so distant it cannot be seen with the naked eye.

Photo Credit: NASA

By L.E. Terry
At nearly 17 times the size of earth, Neptune is the fourth-largest planet as well as one of the slowest moving. Lying at the outer reaches of the solar system, Neptune trudges through space, taking165 earth years to orbit the sun.

Discovery and History

It was Italian scientist and scholar Galileo Galilei who first observed Neptune, spotting the planet in 1613. He was only able to glimpse the planet for two nights, and was thus unable to gain a clear understanding of its movements. Because of this lack of information, Galileo mistook the planet for a star.

When Neptune's identity was finally uncovered in 1846, it was because of the discovery of another planet, Uranus. Astronomers had noted that Uranus did not behave as it should according to Newton's Laws of Motion. The reason, it was thought, must be that some other planet was exerting an influence on Uranus' orbit. Two astronomers, working independently, predicted the location of the unknown planet based on observations of Jupiter, Saturn and Uranus. The calculations of these astronomers, John Couch Adams and Urbain J.J. Leverrier, were confirmed when Urania Observatory director Johann G. Galle and his assistant Heinrich L. d'Arrest observed the planet very near to their predicted locations on Sept. 23, 1846. What followed was an international dispute, with both the English and the French seeking credit and the right to name the new planet. Today, both countries, and both astronomers, are credited with the discovery.

Size and Location

Neptune is the eighth planet from the sun, situated nearly 2.7 million miles away from the star it orbits. It is 30 times farther from the sun than is Earth. Because it orbits the sun so slowly, Neptune has not yet made a complete circle around the sun since its discovery. Neptune's orbit is at times interrupted by Pluto, whose unusual elliptical orbit periodically causes it to cross into Neptune's path, making Neptune the farthest planet from the sun for a 20-year period once every 248 Earth years.

Though massive, Neptune has the smallest diameter of the solar system's "gas giants," a group of planets which also includes Jupiter, Saturn and Uranus, and so named because they lack any solid surfaces. Still, Neptune is significantly larger than Earth, with almost four times its diameter and a volume that could hold 60 Earths. Though heavier than Earth, it is not as dense.

Composition and Atmosphere

Neptune appears to be composed mainly of hydrogen, helium and water, as well as silicates, the minerals that make up most of Earth's rocky surface. Inside, the planet seems to consist of heavily compressed gases that combine to form a liquid layer encompassing Neptune's inner core of rock and ice.

Thick, rapidly moving clouds surround Neptune, swirling around the planet at around 700 miles per hour. The clouds farthest from the planet's surface are primarily frozen methane, and the darker clouds, lying below the methane clouds, are believed to consist of hydrogen sulfide. Neptune's atmosphere contains primarily hydrogen and helium, with some methane. It is this methane that contributes to the planet's blue color, produced when red light is absorbed by the methane.

There has been one mission to Neptune: the Voyager 2 spacecraft, which visited the planet on Aug. 25, 1989. Voyager 2 found something odd on the planet's southern hemisphere: a dark area similar in appearance to a hurricane, made up of violently rotating masses of gas. Resembling Jupiter's Great Red Spot, but measuring about half the size, the area was nicknamed the Great Dark Spot. The spot moved at about 700 miles per hour, pushed westward by Neptune's high winds. In 1994, however, observations by the Hubble Space Telescope revealed that the Great Dark Spot had disappeared. It is not known if the spot has dissipated or is merely hidden by something else in the planet's atmosphere. A few months after the discovery of the spot's disappearance, a new spot was identified, this time in the planet's northern hemisphere, indicating that Neptune's atmosphere is probably continuously changing. Two other features were discovered by Voyager 2. There was an additional, smaller dark spot in the southern hemisphere, as well as a small, uneven white cloud. This cloud, nicknamed "The Scooter," races around Neptune about every 16 hours. Exactly what "The Scooter" is is not certain, but it may be a plume that rises from somewhere deep in the atmosphere.

Satellites and Rings

Of Neptune's 13 known satellites, six were discovered by Voyager 2. The largest of Neptune's moons is Triton, which was discovered only 17 days after the planet. Measuring about 1,681 miles in diameter, Triton is located nearly 220,440 miles from Neptune. Traveling in a circular orbit around Neptune every six days, Triton is the only major satellite in the solar system that orbits in the opposite direction of its planet. It is believed Triton may have originally been a comet that traveled around the sun and was eventually drawn in by Neptune's gravitational pull. At -390 degrees Fahrenheit, Triton's surface temperature is the coldest known in the solar system. There is evidence that volcanoes once showered Triton with a mixture of water and ammonia, which is now frozen on the satellite's surface. Some of Triton's volcanoes remain active, however, emitting nitrogen ice crystals up to six miles above its surface. Triton is circling closer and closer to Neptune, and it is believed it will collide with the planet in anywhere from 10 million years to 100 million years. As it orbits the planet, Triton forms immense rings around Neptune that will eventually equal or even surpass the complex ring system that surrounds Saturn.

Neptune is encircled by three prominent rings and one faint ring, all of which are dark and appear to be composed of dust particles. The outermost ring, Adams, consists of three bright arcs, named Liberty, Equality and Fraternity, in which the dust material appears to be clumped. The other rings are Leverrier, which has two outer extensions named Lassell and Arago; an unnamed ring; and Galle, a faint but broad ring.

Viewing Neptune

Too distant to be seen with the naked eye, Neptune can only be viewed with the aid of a telescope. Even then, detailed charts, like those made with a planetarium software program, may be necessary in order to pinpoint the planet's specific location.